1. Field of the Invention
The present invention relates to the fabrication of solid-state electrooptical devices and more particularly to a method of fabricating transparent metal oxide electrodes on such devices.
2. Discussion Related to the Problem
Solid-state electrooptical devices, such as charge coupled device (CCD) image sensing arrays, in which light passes through, or around a gate electrode on the device, are well known. In the earliest of such devices, the gate electrodes were aluminum and therefore opaque. Small gaps were left between the electrodes through which light could pass. The presence of these small gaps caused serious problems in manufacturing the devices and imposed limitations on the performance of the devices. Shorts frequently occurred across the narrow gaps between the electrodes, rendering the devices useless. The presence of the bare gaps caused fringing fields in the devices that reduced their charge transfer efficiency. Finally, the relatively small light responsive surface area of the devices severely restricted their light gathering efficiency.
An improvement in the manufacturability and performance of front illuminated CCD image sensing arrays was achieved by the use of semitransparent polysilicon as the electrode material. The article entitled "Charge Coupled Area Image Sensor Using Three Levels of Polysilicon" by C. H. Sequin et al, IEEE Transactions on Electronic Devices, Vol. ED 21, No. 11, Nov. 1974, describes a three-phase CCD image sensor using three levels of semitransparent polysilicon electrodes. With the semitransparent electrode material, gaps between the electrodes were no longer required to allow light to enter the device from the front side. As a result, the electrodes were allowed to slightly overlap (with an interposed layer of insulating material such as SiO.sub.2) thereby reducing the probability of shorts between the electrodes, and greatly improving the transfer efficiency of the device by reducing the fringing fields caused by gaps between the electrodes.
The polysilicon electrodes still had the disadvantage that they were only semitransparent, some of the light reaching the surface of the device was absorbed by the electrode structure and therefore the light gathering efficiency of the device was limited. A proposed solution to this problem was to use a more transparent conductive material such as indium oxide or tin oxide for the electrodes. Unfortunately, there are several problems associated with the use of a transparent conductive metal oxide for the electrodes in an electrooptic device. For one thing, conventional photoresists may not adhere at all well to the metal oxide layers in the presence of the concentrated or hot acids commonly required to etch these layers, making patterning by photolithography difficult. A second problem is that the contacts between the transparent metal oxide electrode structure and the final metal layer on the bonding pads (e.g. aluminum) tend to form insulating oxide interfaces (e.g. aluminum oxide) at the bond due to the reduction of the transparent metal oxide by the aluminum, thereby destroying electrical contact to the electrodes. A third problem is the need to pattern the final metallization layer without affecting the transparent metal oxide electrodes, in light of the fact that the transparent metal oxide electrodes are attacked by conventional metal etchants in the presence of a strongly reducing metal such as aluminum.
U.S. Pat. No. 3,941,630 issued to R. D. Larrabee on Mar. 2, 1976, discloses one method of patterning transparent metal oxide electrodes in a CCD image sensing device. According to the method disclosed by Larrabee, a transparent metal oxide layer is deposited on the surface of the device, and the metal oxide layer is covered with a thin layer of aluminum. The aluminum is patterned using photolithographic techniques and an etchant that does not substantially attack the transparent metal oxide. The remaining aluminum is anodized, leaving a transparent insulating pattern of aluminum oxide on the transparent metal oxide. The aluminum oxide pattern is then employed as a resist to pattern the transparent metal oxide, leaving a pattern of transparent metal oxide electrodes covered by an insulating layer of transparent aluminum oxide. While providing a method for patterning transparent metal oxide electrodes, this method does not solve the second problem noted above. That is, the problem of forming contacts between the metal oxide electrodes and the final metal layer on the bonding pads of the device. Rather, the method leaves an insulating layer of aluminum oxide covering the transparent metal oxide electrodes.